1. Field of the Invention
This invention relates to a process and apparatus for the recovery of starch and gluten from a wheat-flour dough.
2. Description of the Prior Art
Starch has been traditionally produced by extraction from tubers and also from corn and rice by disintegrating the tubers or grain and then washing out the starch from the pulp on screens of various designs. The production of wheat starch, on the other hand, requires a totally different process. The reason for this is the rather special physical properties of wheat gluten. The gluten tends to swell during the mixing process of wheat flour with water and forms a network of a cellular pattern in which the gluten forms the cell wall and the starch fills the cells. The hydrated or swollen gluten is very elastic and this property is utilized when leavening the dough for bread baking. When the yeast converts some of the starch granules into alcohol, carbon dioxide is formed and expands the cells. The elastic gluten walls prevent the gas from escaping and a fluffy and easier digestible bread can be baked.
While this is obviously a very beneficial feature of wheat flour in terms of bread production, it is at the same time one of the most serious problems in the utilization of wheat flour for the production of starch. When separating the gluten and starch from the wheat dough, water is used as the vehicle for washing the starch away from the gluten and in the unwashed dough, the gluten is dispersed as a very thin network. While the starch particles have a definite size and shape, the gluten has neither of these characteristics. Wheat gluten is, however, very sticky, tough and plastic in relation to gluten particles from other flour sources, which property makes it possible to collect the wheat gluten in the form of gluten lumps on a screen while the starch is washed through the screen by means of a washing liquid.
The above characteristics of wheat flour dough mean that in the separation process two contradictory conditions must be taken into consideration. Thus, on the one hand, the washed gluten particles or strands should accumulate as quickly as possible in order that the gluten losses may be kept small, while on the other hand, the gluten network must be ruptured in order that starch granules embedded in this network will come into contact with the washing liquid. These contradictory conditions have necessitated labour consuming and very lengthy and expensive procedures while obtaining wheat starch of rather poor quality and yields.
Continuous processes have been developed which involve a quite complete dispersion of the dough in wash water and the recovery of the gluten on vibrating or rotating screens having a very fine mesh of 100 microns or finer. These screens obviously require much cleaning and maintenance and, due to the large screen area required, the contact between products and air is considerable, this resulting in undue and unhygienic bacterial action and breakdown especially of the gluten. Moreover, this system requires a comparatively "hard" type of flour.
A significant advance in the production of starch and gluten from wheat flour dough is the system described in Plaven U.S. Pat. No. 3,669,739 issued June 13, 1972. In that system a wheat flour dough is simultaneously mechanically treated and spray washed with a starch absorbing liquid, this liquid being quickly removed from the recovery region so as to prevent an undue buildup of wash liquid. The dough is subjected to repeated pinching, shreading and cutting actions by the disposition and use of improved scrapers, and spreading members revolving with a rotatable axle extending axially within a perforated semicircular trough-shaped treatment zone. The purpose of this is to break down the gluten network of the dough so that the starch can be washed out and the starch and reformed gluten lumps recovered in different regions.
While that system has many advantages, it also has some quite severe commercial disadvantages. One of the major disadvantages is that the lumps of dough are subjected to repeated and progressive steps of pushing and moving, pinching, cutting and shreading operations by a plurality of different axially spaced and circumferentially offset radially extending pushing paddles and cutting sabres, the latter members operating by pinching the dough lumps between the radially outermost edges thereof and the trough bottom. This pushing and pinching of the dough against the perforated screen by the action of the various treatment members tend to cause the dough to block the screen thereby decreasing the efficiency of the system.
It is the object of the present invention to overcome the above disadvantages of the prior art systems.